To provide security against intrusion, many residential and commercial buildings, factories, parking garages, etc., use security systems. Such systems typically include motion and/or infrared sensors located throughout a facility, window and door detectors that indicate whether a door or window is open or shut, and one or more central panels that interface directly to these “supervised inputs.”
FIGS. 1A and 1B are schematics of circuits typically used in supervised inputs such as window and door detectors.
FIG. 1A illustrates a typical normally-open switch circuit 10. When the switch 16 is open, only resistor R1 is seen, so the circuit 10 has a resistance of 2 k ohms. When the switch 16 is closed, resistor R1 is in parallel with resistor R2, yielding a net resistance of 1 k ohms. Thus, a resistance of 1 k ohms indicates that the switch is closed, while a resistance of 2 k ohms indicates that the switch is open.
FIG. 1B illustrates a typical normally-closed switch circuit 20. In this circuit 20, when the switch 26 is open, resistors R3 and R4 are in series, yielding a total resistance of 4 k ohms. On the other hand, when the switch 26 is closed, resistor R3 is shorted out, so the total resistance is from R4, i.e., 2 k ohms. Thus, a resistance of 2 k ohms indicates that the switch is closed, while a resistance of 4 k ohms indicates that the switch is open.
The resistive circuits 10, 20 of FIGS. 1A and 1B respectively also ensure that if an intruder were to physically cut the wires leading to the detector or short them together, the resulting resistance would approach infinity or zero ohms and would thus be detectable as a breach of the system.
Of course, the exact resistances may be different from the provided examples. In fact, one vendor's detectors typically are different from another's. For example, R1 may be 6 k ohms and R2 might be 18 k ohms. This can be a problem when a user (e.g., the owner of a house or the management of a facility) wants to upgrade to a new system (e.g., another vendor) and systems are incompatible. The expense of replacing all of the sensors, which may be buried in walls or even in concrete, may be extremely high.
In other words, when an existing access control system is replaced with another access control system, for example, that of another manufacturer, or even a different system of the same manufacturer, the various input switches that are already installed in the building may not be compatible with the new equipment. Prior to the present invention, an installer would typically be required to open each sensor and manually replace the termination resistors with the values expected by the new equipment. This can be a very labor intensive and time consuming process.